EP2360003B1 - Method and device for operating a needle valve nozzle of an injection moulding tool - Google Patents

Description

The invention relates to a method for controlling a needle valve nozzle of an injection molding tool according to the preamble of claim 1, and a device for carrying out the method according to the preamble of claim 6 and 8. In the field of plastic processing, today needle valve nozzles are used to manufacture plastic molded parts with which Injected via a melt material distributor and liquefied by heating plastic is injected into an injection mold, also known as a cavity, from which the molded part can be removed after the plastic has solidified. Methods and devices for operating needle valve nozzles are known from the EP 2 226 173 A1 , of the JP 6 064002 A , of the US 2002/121713 A1 and the JP H11 42683 known. The EP 2 226 173 A1 claims priority from 06.03.2009, was published on 08.09.2010 and represents a state of the art according to Article 54 (3) EPC. It discloses a piston-cylinder drive for needle valve nozzles, the pressure medium outflow from the piston opening when the nozzle is opened. Cylinder drive is throttled. The JP 6 064002 A also discloses throttling the pressure medium outflow when the valve pin is opened. The US 2002/121713 A1 discloses the use of hydraulic servo valves or electric motors for valve needles. The US 2002/121713 and the JP H11 42683 reveal conical sections of the nozzle and valve pin. Usually, several needle valve nozzles are used, each containing a valve needle, which is moved in the axial direction by an actuating device in the form of a double-acting hydraulic cylinder in order to open or close an opening in the housing of the valve gate nozzle, through which the plastic melt is injected into the cavity close.

If several valve gate nozzles are opened at the same time, the problem arises that when the plastic melt flows introduced through the nozzles flow into the cavity, a weld line can occur between the melt fronts flowing into one another, which usually leads to visual defects and a mechanical weak point in the finished molded part .

In a method also known in the prior art as cascade injection molding or sequential injection molding, the weld line is avoided in that the The respective needle valve nozzles can only be actuated one after the other if the melt front is already in front of its opening or has moved past it, whereby the front of the plastic melt is taken over by the melt flow from the subsequently opened needle valve nozzle.

The problem here is that the high injection pressure required for the reliable injection of the plastic melt builds up at the beginning on the open needle valve nozzles. This injection pressure is also present at the still closed valve gate nozzles. In the known needle valve nozzles, in which the actuation of the valve needles occurs suddenly and within a very short period of only a few tenths of a second between the closed position and the open position without measurable intermediate steps, the pressure present at the already opened valve gate nozzles in the delivery system the plastic melt drops suddenly when one or more further needle valve nozzles are opened.

This brief sudden drop in pressure in the plastic melt already flowing into the cavity in turn leads to a brief stagnation of the front of the melt in the cavity, which disadvantageously often results in optical defects and mechanical weakening in the finished molded part.

Accordingly, it is an object of the present invention to provide a method with which a large fluctuation in the pressure at the valve gate nozzles can be avoided. Another object of the present invention is to provide an apparatus for performing the method.

This object is achieved according to the invention by a method and a device with the features of claims 1, 6 and 8.

Further features of the invention are contained in the subclaims.

According to the invention, an injection molding tool for injecting a plastic melt into a cavity of an injection mold comprises one or more needle valve nozzles, each of which has a nozzle housing in which one Closure needle with a conical section can be moved by an actuating device between a closed position in which the flow of the plastic melt is interrupted by the opening of the needle shut-off nozzle and an open position in which the liquid plastic melt is injected into the cavity.

The method according to the invention is characterized in that the size of the volume flow of the plastic melt is controlled during the injection process.

The use of the method according to the invention has the advantage that an excessive pressure drop, which arises due to the nozzle openings in the entire supply system for the plastic melt being released too quickly at the beginning of the opening process of the respective needle valve, can be specifically prevented or at least reduced in that the The speed of the respective closure needle at the beginning of the opening movement is preferably reduced compared to the speed at the end of the opening movement.

Another advantage of the method according to the invention can be seen in the fact that, by avoiding stagnation of the melt front in the cavity, injection molded products can be produced with a higher quality.

In the method according to the invention, the valve pin has an opening angle in the range between 1 ° and 20 ° with respect to the longitudinal axis of the valve pin, and the nozzle housing has an opening angle between 10 ° and 40 °. In conjunction with a reduced and preferably constant speed of movement of the closure needle during the opening process, this leads to a further reduction in pressure fluctuations in the plastic melt and to an increase in the length of the annular gap through which the melt flows at the beginning of the opening process, which in turn has an advantageous effect on maintaining the chemical consistency of the plastic melt affects.

Here, the duration of the opening process, which in conventional needle valve nozzles is in the range of 0.1 to 0.2 seconds for the full stroke of the valve needle, can be extended to a total duration of several seconds, preferably 2 to 6 seconds, in the method according to the invention, whereas the closing process has a duration of preferably 0.1 to 0.3 seconds.

This has the advantage that the cross section of the opening released by the closure needle increases comparatively slowly and uniformly when the closure needle is moved from the closed position into the open position at the beginning of the injection process, as a result of which the volume flow of the plastic melt in the opening Needle valve nozzle increases slowly. In other words, the conical wall sections of the shut-off needle and the nozzle housing move apart evenly when the shut-off needle is moved from the closed position into the open position at the beginning of the movement over the entire length of the conical section, which has the effect that the air flow through the released opening extends over one compared to known needle valve nozzles extends longer length of the valve needle. This advantageously avoids a slit-shaped, sharp-edged opening which occurs in the known needle valve nozzles at the beginning of the opening process and whose expansion, viewed in the longitudinal direction of the valve needle, is limited only by the expansion of the annular edge of the nozzle housing in the region of the injection opening. Such a sharp-edged cross-sectional opening, in conjunction with the comparatively high injection pressure of the plastic melt, easily leads to a disadvantageous chemical change in the plastic melt, which in turn can lead to weak points or visual defects in the molded part.

In the preferred embodiment of the method according to the invention, the actuating device comprises a piston which is pressurized with a pressure fluid, in particular hydraulic oil, from an associated pressure fluid source and which is movably received in a corresponding cylinder in a known manner. In order to reduce the speed of movement of the piston when opening the valve pin, the Pressure fluid arranged a preferably adjustable flow throttle, which flows through the pressure fluid and reduces the volume flow of the pressure fluid when moving the valve pin from the closed position to the open position.

The advantage of using a flow restrictor is that it can also be retrofitted comparatively inexpensively to existing plastic injection devices, and in the case of a restrictor with e.g. the movement speed of the shut-off needle during the opening process can be set separately for each needle without great effort by means of an adjusting screw which is adjustable from the outside.

The throttle is further preferably assigned a bypass line with a check valve contained therein, via which the pressure fluid is guided when the valve needle moves from the open position to the closed position. In this embodiment of the invention, the pressure fluid flows over the throttle when the valve pin is opened and over the opened check valve and the bypass line downstream in terms of flow and running parallel to the throttle valve when the valve needle is closed.

This advantageously enables the use of a comparatively inexpensive adjustable flow restrictor, e.g. an adjusting screw extending into the channel of the supply or return line, with which the flow cross-section of the channel can be changed, both slowing the opening of the needle valve and simultaneously accelerating the closing of the valve valve.

As an alternative to the use of a flow restrictor, a proportional valve can also be integrated into the pressure fluid line, through which the pressure fluid flows when the needle valve nozzle is opened and which preferably controls the volume flow of the pressure fluid according to a predetermined profile, which, for example, in the form of a map in an electronic control device controlling the proportional valve can be filed.

According to a further concept on which the invention is based, it can be provided here that the throttle or the proportional valve can be adjusted hydraulically, pneumatically, electrically and / or mechanically, as a result of which the opening time of the needle valve nozzle associated with the throttle or the proportional valve can advantageously be adjusted easily and time-saving way.

Alternatively, it can be provided that the actuating device comprises a servomotor mechanically coupled to the locking needle, and that an electronic control device is provided which controls the servomotor when the locking needle moves from the closed position into the open position in accordance with a predetermined speed profile. Although the use of a servo motor is comparatively complex in terms of control technology, the position and speed of the valve pin can be adjusted with high precision in a very short time.

According to a further concept on which the invention is based, a device for carrying out the method according to the invention comprises a nozzle housing, in which a closure needle with a conical section is moved by an actuating device from a closed position, in which the flow of the plastic melt is interrupted, to an open position, in which the liquid plastic material is injected into the cavity, is movable. The actuating device in this case has a double-acting piston which is guided in a cylinder and is charged with a pressure fluid from a pressure fluid source, in particular a hydraulic source. Arranged in the circuit of the pressure fluid is a flow restrictor, also referred to below as a throttle, which reduces the volume flow of the pressure fluid when the actuating device moves the closure needle from the closed position into the open position.

In the preferred embodiment of the invention, the preferably adjustable throttle is assigned a bypass line with a check valve arranged therein, via which the pressure fluid is passed when the piston moves the valve pin into the closed position. Alternatively, instead of a check valve and a bypass line, a multi-way valve can also be used, which switches the flow paths of the pressure fluid directly.

By using a bypass line, the full line cross-section is available for the flow of the pressure fluid when the valve needle is moved into the closed position without constrictions interfering with the flow of the pressure fluid. As a result, the time period in which the closure needle is moved into the closed position can advantageously be shortened given a predetermined size of the piston and a predetermined pressure of the pressure fluid.

According to a further idea on which the invention is based, the throttle is arranged in the line of the pressure fluid, through which the pressure fluid is led out when the valve pin is opened from the corresponding chamber of the double-acting cylinder, for example to an expansion tank. This has the advantage that the second chamber of the double-acting cylinder, into which the pressure fluid is introduced to open the needle valve, is acted upon from the start with the maximum working pressure provided by the pressure fluid source.

According to an alternative embodiment of the invention, the actuating device can comprise a servo motor mechanically coupled to the locking needle, to which an electronic control device is assigned, which controls the servomotor when the locking needle moves from the closed position into the open position according to a predetermined speed profile.

This embodiment advantageously enables a computer-controlled selection of different speed profiles, which can be used in particular in the production of different injection molded parts. For example, after a first injection molding process carried out on a test basis, the duration of the opening process can be adapted for some selected needle valve nozzles be made. It is advantageous here that the setting of the speed profiles with which the servomotors are controlled can, for example, be carried out centrally on a computer, which preferably also contains a database in which a large number of speed profiles for the respective locking needles are stored, which are used for production of different molded parts can be used.

In the device according to the invention, the conical inner section of the housing of the needle valve nozzle comprises a first section, which is assigned to the conical section of the valve needle in the closed position and which has an opening angle between 2 ° and 25 ° with respect to the longitudinal axis of the valve needle, whereas the nozzle housing has a relative total angle in Has a range of 10 ° - 40 °.

Here, the conical section of the valve pin has an opening angle between 1 ° and 20 °, and the housing of the needle valve nozzle has an inner cone with an opening angle of 2 ° - 25 in the area of the conical section of the valve pin in the closed position with respect to the axis of the valve pin °.

In this way, in particular in connection with a previously described, preferably mechanically adjustable flow restrictor in the return line, a device is created with which the volume flow increases comparatively slowly at the beginning of the opening process of the needle valve nozzle and the occurrence of sharp edges in the released area through which the plastic melt flows the injection opening is avoided.

It can also be advantageous here if a closure needle with an opening angle that deviates from the conical section of the nozzle housing is used, since this causes the temporal change in the cross-sectional area through which the plastic melt flows in a predetermined time interval when opening and moving the closure needle, which preferably moves at a constant speed the wise is changed that the volume flow of the injected plastic melt increases very slowly at the beginning.

In other words, in the case of the invention, the conical section of the closure needle interacts with the conical inner geometry of the nozzle housing when the closure needle is moved at an essentially constant speed in such a way that the volume flow - and thus the pressure drop in the plastic melt - is very low at the beginning of the opening process is and also only increases very slowly, and the volume flow subsequently increases disproportionately to the distance covered in the course of the opening movement of the closure needle.

The disproportionate increase in the volume flow of the injected plastic melt can be further increased by the fact that the first conical section of the nozzle housing, which interacts with the conical section of the sealing needle, has an opening angle of, for example, 1 ° to 20 ° larger than the angle of the conical section of the sealing needle , which is advantageously in the range from 2 ° to 25 °.

In a corresponding manner, the first conical section of the nozzle housing is followed by a corresponding second conical section of the nozzle housing, which has an opening angle adapted to the opening angle of the first section, so that the two conical sections together have a total opening angle of 10 ° -40 °.

The invention is described below with reference to the drawings based on a preferred embodiment of the device.

Fig. 1

eine schematische Darstellung der bevorzugten Ausführungsform der erfindungsgemäßen Vorrichtung,

Fig. 2

eine schematische Darstellung einer weiteren Ausführungsform der erfindungsgemäßen Vorrichtung, und

Fig. 3

eine schematische Schnittflächendarstellung des konischen Abschnitts der Verschlussnadel und des Düsengehäuses in geöffnetem Zustand.

In the drawings:

Fig. 1

2 shows a schematic representation of the preferred embodiment of the device according to the invention,

Fig. 2

a schematic representation of a further embodiment of the device according to the invention, and

Fig. 3

is a schematic sectional view of the conical section of the valve pin and the nozzle housing in the open state.

As in Fig. 1 A device 100 according to the invention for injecting a plastic melt into a cavity of an injection mold (not shown in more detail) comprises a needle valve nozzle 1, which has a nozzle housing 22 in which a valve needle 8 is moved by an actuating device 4 along an axis 18 from a closed position into an in Fig. 3 shown open position is movable back and forth to a in Fig. 3 to open or close the injection opening 3 shown. The actuating device 4 comprises a cylinder 11, in which a double-acting piston 12 is received, which is mechanically coupled to the closure needle 8.

According to the representation of Fig. 1 the interior of the cylinder 11 is divided by the piston 12 into a first chamber 10a on the needle side and a second chamber 10b, each of which can be pressurized with a pressure fluid 26, such as a hydraulic or pneumatic medium, via its own connection 7a and 7b. For the sake of simplicity, a hydraulic medium 26 is assumed below.

To move the valve pin 8 from the closed position into the open position, the hydraulic medium 26, which is provided at high pressure by a hydraulic source, not shown in detail, in particular a hydraulic pump, is provided. Via a feed line 14 and a multi-way valve 2 fed to the first needle 7a on the needle side, via which it flows into the first chamber 10a on the needle side. In this case, the associated outlet 9a of the multi-way valve 2 is connected directly to the first connection 7a via a corresponding hydraulic line, in order to allow the hydraulic medium 26 to flow as freely as possible.

In contrast, the second port 7b of the cylinder 11 is connected to the second outlet 9b of the multi-way valve 2 via a flow restrictor 5 and a check valve 6 arranged parallel to it in a bypass line 28, the check valve 6 being arranged in the bypass line 28 in such a way that the bypass line 28 is blocked and the hydraulic medium 26 flows from the second chamber 10b via the flow restrictor 5 to the multi-way valve 2 when the first chamber 10a is pressurized to move the valve pin 8 from the closed position to the open position. Here, the flow restrictor 5 can comprise an adjusting screw, not shown in the drawings, with which the line cross section of the line, in which the flow restrictor is accommodated, can preferably be changed continuously, by the volume flow of the hydraulic medium 26 - and thus the movement speed of the piston 12 and to be able to reduce the locking needle 8 coupled to it to a desired value.

During the closing process of the needle valve nozzle 1, which is initiated by a corresponding switchover of the multi-way valve 2, the hydraulic medium 26 supplied from the hydraulic source via the feed line 14 is directly supplied to the bypass line 28 and the flow restrictor 5 by a corresponding position of the multi-way valve 2, as a result of which the Check valve 6 opens and the hydraulic medium 26 flows past the throttle 5 into the second chamber 10b and the piston 12 together with the locking needle 8 coupled to it in the direction of the in Fig. 3 indicated injection opening 3 to move. At the same time, the first chamber 10a of the cylinder 11 is connected to the return line 16 via the multi-way valve 2 connected, via which the hydraulic medium 26 flows back, for example, into a reservoir, not shown.

This ensures that when the needle valve nozzle 1 is closed, the entire pressure of the hydraulic medium 26 present in the feed line 14 is used and the speed of movement of the piston 12 and the valve needle 8 coupled therewith is reduced only when the valve valve nozzle is opened by the flow restrictor 5.

In Fig. 2 An alternative embodiment of the device according to the invention is shown, in which the throttle 5 is arranged in a line section downstream of the multi-way valve 2 as viewed from the piston 12. In this embodiment, the bypass line 28 does not have a check valve 6, but the flow of the hydraulic medium 26 through the bypass line 28, which is connected to the multi-way valve 2 via its own connection, is controlled by the multi-way valve 2 - which here is preferably a 5/2 -Multi-way valve designed, is - released or blocked.

According to the representation of Fig. 2 In this embodiment of the invention, the supply line 14 is connected directly to the connection 7a when the needle valve nozzle 1 is opened, and the outlet of the multi-way valve 2 assigned to the bypass line 28 is blocked. At the same time, the connection 7b of the second chamber 10b of the cylinder 4 is connected in terms of flow to the line section in which the flow restrictor 5 is arranged, so that the hydraulic medium 26 flows through the restrictor 5 into the return line 16 and thereby the volume flow of the hydraulic medium 26 is reduced accordingly. Since this also reduces the speed of movement of the piston 12 and the locking needle 8 coupled to it, the opening cross section of FIG Fig. 3 Injection opening 3 shown correspondingly slower, which leads to a reduction in the pressure drop in the supply system for the liquid plastic melt.

The shape of the shut-off needle 8 and the associated nozzle housing 22 will be described below with reference to FIG Fig. 3 described, which shows a sectional view of the conical section 20 of the valve pin 8 and the nozzle housing 22 at the level of the longitudinal axis 18 of the valve pin 8.

Like the representation of Fig. 3 It can be seen in detail here that the conical section 20 of the closure needle 8 has a relative total angle α in the range from 1 ° to 20 °. The angle α is spanned by the longitudinal axis 18 of the closure needle 8 and the dashed straight line running through the two outer corner points A and B of the conical section 20.

The nozzle housing 22 has as in FIG Fig. 3 shown a first conical section 24, which has an opening angle β in the range from 2 ° to 25 ° and with the dotted straight line through the corner points C and E in the drawing of FIG Fig. 3 is shown. This first conical section 24 interacts with the conical section 22 of the shut-off needle 8 in such a way that the injection opening 3 of the needle shut-off nozzle 1 is closed by the shut-off needle 8 in a closed position (not shown) and the opening cross section of the needle shut-off nozzle 1 at the start of the movement of the shut-off needle 8 changes very slowly from the closed to the open position and, from the point at which the closure needle 8 has left the region of the conical section 24, increases more rapidly with the distance traveled by the closure needle 8.

The first conical section 24 of the nozzle housing 22 is followed by a second conical section 25 which adjusts the maximum opening cross section of the first conical section 24 to the cylindrical cross section of the nozzle housing 22. The angle of the second conical section 25 of the nozzle housing 22 is selected such that the first and second conical sections 24, 25 together have a total opening angle γ in the range from 10 ° to 40 °. This total opening angle γ is shown in the representation of Fig. 3 between the Axis 18 and the dash-dotted straight line shown by points C and D are defined.

List of reference numbers

1

Valve gate

2nd

Multi-way valve

3rd

Injection port

4th

Actuator

5

throttle

6

check valve

7a

first connection

7b

second connection

8th

Valve pin

9a

first outlet of the multi-way valve

9b

second outlet of the multi-way valve

10a

first chamber

10b

second chamber

11

cylinder

12th

piston

14

Supply

16

Return line

18th

Axis of the valve pin

20th

conical section of the valve pin

22

Nozzle housing

24th

first conical section of the nozzle housing

25th

second conical section of the nozzle housing

26

Pressurized fluid / hydraulic medium

28

Bypass line

100

device according to the invention

α

angle

β

angle

γ

angle

A

Point / corner point

B

Point / corner point

C.

Point / corner point

D

Point / corner point

E

Point / corner point

Claims (8)

1. Method for injecting a molten plastic into a cavity of an injection mould, with a needle valve nozzle (1), which has a nozzle casing (22), in which a shut-off needle (8) with a conical portion (20) can be moved by an actuating device (4) between a closed position, in which the flow of the molten plastic is interrupted, and an opened position, in which the liquid molten plastic is injected into the cavity, wherein the volumetric flow of the molten plastic is controlled during the injection process,
   characterized
   in that the speed of movement of the shut-off needle (8) is lower during the opening operation than during the closing operation and
   in that the nozzle casing (22) has a conical inner portion, which comprises a first conical portion (24), which is assigned to the conical portion (20) of the shut-off needle (8) and interacts with the latter in the closed position and which has with respect to the longitudinal axis (18) of the shut-off needle (8) an opening angle of between 2° and 25° and is adjoined by a second conical portion (25), wherein
   the conical portion (20) of the shut-off needle (8) has a relative total angle (α), defined by the longitudinal axis (18) of the shut-off needle (8) and a straight line running through the two outer corner points (A, B) of the conical portion (20), of between 1° and 20°, and the first and second conical inner portions (24, 25) of the nozzle casing (22) together have with respect to the longitudinal axis (18) a total opening angle (γ) in the range of 10° - 40°.
2. Method according to Claim 1,
   characterized
   in that the actuating device (4) comprises a piston (12) acted upon by a pressure fluid (26) from a pressure fluid source, and in that the circuit of the pressure fluid (26) is assigned a throttle (5) or a proportional valve, which reduces the volumetric flow of the pressure fluid (26) during the movement of the shut-off needle (8) from the closed position into the opened position.
3. Method according to Claim 2,
   characterized
   in that the throttle (5) is assigned a bypass line (28) with a nonreturn valve (6) contained therein, by way of which the pressure fluid (26) is passed during the movement of the shut-off needle (8) from the opened position into the closed position.
4. Method according to Claim 2 or 3,
   characterized in that the throttle (5) or the proportional valve is hydraulically, pneumatically, electrically and/or mechanically adjustable.
5. Method according to Claim 1,
   characterized in that the actuating device (4) comprises a servomotor mechanically connected to the shut-off needle (8), and in that an electronic control device is provided, which controls the servomotor during the movement of the shut-off needle (8) from the closed position into the opened position according to a predetermined speed profile.
6. Device for carrying out the method according to Claim 1, with a needle valve nozzle (1), which has a nozzle casing (22), in which a shut-off needle (8) with a conical portion (20) can be moved by an actuating device (4) between a closed position, in which the flow of the molten plastic is interrupted, and an opened position, in which the liquid molten plastic is injected into the cavity, wherein the actuating device (4) comprises a double-acting piston (12) acted upon by a pressure fluid (26) from a pressure fluid source,
   wherein a throttle (5) is assigned to the circuit of the pressure fluid (26), which reduces the volumetric flow of the pressure fluid (26) during the movement of the shut-off needle (8) from the closed position into the opened position, characterized
   in that the nozzle casing (22) has a conical inner portion, which comprises a first conical portion (24), which is assigned to the conical portion (20) of the shut-off needle (8) and interacts with the latter in the closed position and which has with respect to the longitudinal axis (18) of the shut-off needle (8) an opening angle of between 2° and 25° and is adjoined by a second conical portion (25), wherein
   the conical portion (20) of the shut-off needle (8) has a relative total angle (α), defined by the longitudinal axis (18) of the shut-off needle (8) and a straight line running through the two outer corner points (A, B) of the conical portion (20), of between 1° and 20°, and the first and second conical inner portions (24, 25) of the nozzle casing (22) together have with respect to the longitudinal axis (18) a total opening angle in the range of 10° - 40°.
7. Device according to Claim 6,
   characterized
   in that a bypass line (28) is assigned to the throttle (5) which is opened up by way of a nonreturn valve or a multi-way valve (2).
8. Device for carrying out the method according to Claim 1, with a needle valve nozzle (1), which has a nozzle casing (22), in which a shut-off needle (8) with a conical portion (20) can be moved by an actuating device (4) between a closed position, in which the flow of the molten plastic is interrupted, and an opened position, in which the liquid molten plastic is injected into the cavity, characterized
   in that the actuating device (4) comprises a servomotor mechanically connected to the shut-off needle (8), and in that an electronic control device is provided, by which the servomotor can be controlled during the movement of the shut-off needle (8) from the closed position into the opened position according to a predetermined speed profile in such a way that the speed of the movement of the shut-off needle (8) is lower during the opening operation than during the closing operation, and
   in that the nozzle casing (22) has a conical inner portion, which comprises a first conical portion (24), which is assigned to the conical portion (20) of the shut-off needle (8) and interacts with the latter in the closed position and which has with respect to the longitudinal axis (18) of the shut-off needle (8) an opening angle of between 2° and 25° and is adjoined by a second conical portion (25), wherein
   the conical portion (20) of the shut-off needle (8) has a relative total angle (α), defined by the longitudinal axis (18) of the shut-off needle (8) and a straight line running through the two outer corner points (A, B) of the conical portion (20), of between 1° and 20°, and the first and second conical inner portions (24, 25) of the nozzle casing (22) together have with respect to the longitudinal axis (18) a total opening angle in the range of 10° - 40°.

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